1. FIELD OF THE INVENTION
The present invention relates to a magnetoresistive reproducing head used for playback of magnetic disk apparatuses or magnetic tape apparatuses such as VTRs.
2. DESCRIPTION OF RELATED ART
Recent magnetic recording apparatuses have increased recording capacity. To accomplish this, as an example, the magnetic disk apparatus is made to have a track width as narrow as 5-6 .mu.m. As the size of the apparatus decreases, the media velocity also decreases. In order to detect signals recorded in such narrow areas at a high signal-to-noise ratio, magnetoresistive heads have been developed in place of the conventional inductive heads. As an example, the magnetoresistive head is made in a structure shown in FIGS. 3(a) and 3(b) (see, for example, the Japanese Patent Laid-Open 60-45922 and the Digests of 13th Annual Conference on Magnetics in Japan, 1989, pp. 228). The magnetic head shown in these figures are called a shunt bias magnetoresistive head.
The magnetoresistive head is designed to regenerate a signal from an electric resistance change when a signal field is applied to a magnetoresistive film such as an NiFe film. The electric resistance change is dependent on the magnetic flux but not on speed as in the case of an inductive head. FIG. 3(a) is a plan view for the head, and FIG. 3(b) is a cross-sectional view taken across line A--A' in FIG. 3(a). The magnetoresistive head comprises a lower shield film 2, a lower gap film 3, a magnetoresistive film 4, a shunt bias film 5, an electrode 6, an upper gap film 7, and an upper shield film 8 laminated on a non-magnetic substrate 1. In general, all the films except the lower shield film 2 and the upper shield film 8 are made very thin. The magnetoresistive film 4, for example, is a few 10 nm thick, and the lower gap film 3 and the upper gap film 7 are around 0.2 .mu.m.
In order to allow the magnetoresistive head to play back the high density recorded signals with a high signal-to-noise ratio, the magnetoresistive film 4, the upper gap film 7 and the lower gap film 3 are formed very thin. With such thin films, the under film of the film to be etched is undesirably subjected to etching during patterning. In the worst case, the under film may vanish, particularly when the film to be etched is relatively thick as compared to the thin under film. In the magnetic head shown in FIGS. 3(a) and 3(b), the problem of undesirable etching of the upper gap film 7 and the electrode 6 formed under the upper gap film 7 occurs in the patterning of the upper shield film 8.
FIGS. 4(a) and 4(b) illustrate the undesirable etching or over etching problem that occurs in the patterning of the upper gap film 7 and upper shield film 8, respectively. As shown in FIG. 4(a), when the upper gap film 7 is patterned an undesired etching of electrode film 6 occurs. The thickness of undesired etching of electrode film 6 is indicated by t.sub.1. In FIG. 4(b), the patterning of upper shield film 8 is performed, for example, by ion milling, which causes the upper gap film 7 to be etched undesirably. The thickness of undesirable etching of upper gap film 7 is indicated by t.sub.2. As stated above, should the undesired etching exceed the thickness of the electrode film 6 in the fabrication of the magnetoresistive heads, then the yield will decrease sharply.
In general, the thin film formed under the film to be etched for patterning is adversely subjected to etching because of the film thickness distribution and the etching rate distribution within and between the substrates. In particular, the undesirable etching is significant in physical etching methods, such as an ion milling, which have been frequently used in fabricating magnetic heads. As for the magnetoresistive head, as described above, it is formed of very thin films, and the unwanted etching of the under films tends to lower the yield to a great extent. As the length of time during which the primary film is etched is dependent on the thickness of the film to be processed and the distribution of the ion milling speed, roughly 10-20% of the whole ion milling time of the film to be processed is required. With the upper shield film 8 formed of an Ni--Fe alloy 2 .mu.m thick, an Al.sub.2 O.sub.3 film, if used as the upper gap film 7, will be etched by 150 nm because its ion milling speed is low. This value is substantially equal to the thickness of the upper gap film 7 and it makes the fabrication process margin extremely narrow. As the upper gap film 7 is made thinner with the intention of increasing recording density, the electrode 6 may also be etched and broken down if the worst case situation occurs.